The bipartisan group of senators working on an immigration reform bill haven't decided how to handle the broad swath of workers who come to the U.S. to perform manual labor. The Senate framework talks about creating "an improved process for admitting future workers" but doesn't commit to a guest worker program specifically.

The framework does, however, make it clear that the immigration bill will contain a special guest-worker program for agricultural and dairy workers (who are mostly immigrants, except when portrayed in Super Bowl commercials). In addition, undocumented farm workers who are here already would get an expedited path to citizenship.

So why do farmers get a special deal, when immigrant workers also fill manual labor jobs in industries like construction, healthcare and hospitality?

Here are few of the reasons:

1. Farm Labor Is Hard and Generally Pays Little

When a Colorado farmer tried to hire some locals to work on his farm harvesting onions in October 2011, he thought the poor economy would have Americans snatching up the chance at a job. His first crop of local workers quit after six hours, according to The New York Times.

"Some simply never came back and gave no reason," the Times reported. "Twenty-five of them said specifically, according to farm records, that the work was too hard."

Farm work is one of the most physically grueling jobs out there. The average worker earns $10 per hour and about $10,000 a year, according to Philip Martin, a professor at University of California, Davis with an expertise in immigration and farm labor. Some workers need to move from one farm to another, following the harvests to cobble together a living.

Since the 1960s, Mexican immigrants have taken the place of American workers in the fields, with just a third of present-day farm workers born in the U.S. Of the roughly 2.5 million farm workers hired each year, more than half are undocumented, according to the Department of Labor. Growers put the number closer to 70 percent.

"We don't have access to a domestic labor force," said Kristi Boswell, the director of congressional relations for the American Farm Bureau Federation. "Our jobs are in rural areas, they're seasonal and temporary in nature."

"Also, frankly, it's very hard work," she said. "It's out in the elements, hand labor."

2. Farmers Already Have a Guest Worker Program, But No One Uses It

The idea of creating a guest-worker program for the agricultural sector may be less controversial than for other sectors because one already exists. The H-2A visa is meant to supply foreign-born agriculture workers to farmers who can't find workers born in the U.S.

Unlike some other visa programs, there's no limit on the number of visas that can be given out. However, farmers only use the H-2A visa to hire 2 percent of their workforce. That's 55,000 workers out of roughly 2.5 million. Kristi Boswell, the director of congressional relations for the American Farm Bureau Federation, said that the red tape associated with the program discourages farmers from using it.

"The H-2A program is used by growers out of necessity only," she said. "It's a very burdensome process to even apply to the program." Boswell said that the visa requires extensive recruiting and makes growers provide housing for the workers, an added expense that can be problematic in some areas because of zoning restrictions.

Farmers want a guest-worker program with fewer stipulations, including the ability to hire on a contract or at will. That brings us to our next point.

3. Farmers and Unions Are Playing Ball -- for Now

One of the big changes in a new guest-worker program would be the ability to hire and fire workers at will.

Under the current guest-worker program, workers are tied to a single employer. That's a hinderance for businesses, which have to pay three-quarters of a worker's salary if a person is let go while under contract. If crops are hit with a bad frost and a harvest is ruined, farmers still need to pay out the majority of the contract to the workers.

Surprisingly, the largest farm workers union, the United Farm Workers of America (UFW), does not oppose the hiring and firing change.

The reason? The contracts tie workers to a single employer, under the conditions of their visa. If a worker is under contract with an unscrupulous employer, he or she might be afraid to report abuses for fear of losing their visa, according to Maria Machuca, communications director for UFW.

"They don't have the same flexibility that other workers in the United States have to go to another employer that will treat them better," she said.

However, the union doesn't want to see any other worker rights rolled back in a guest-worker program, Machuca said. "We understand there's a need for legal workforce and we want to make sure we address those issues, but in a way where it doesn't affect the workers themselves, where they continue to have their protections."

4. The Flow of Farm Workers From Mexico Is Slowing Down

When farmers complain that they can't get enough immigrant workers, observers often blame border security and "show me your papers" laws targeting immigrants at the state level. But there's another reason for the labor shortages: Mexican workers -- both in the U.S. and Mexico -- are increasingly less likely to do farm work.

A study by another UC Davis professor, Ed Taylor, shows that fewer Mexicans are working in agriculture. "It's not just the U.S., it's all around the world -- as incomes go up, people stop doing hired farm work," Taylor said.

At the same time, Mexican farms are increasingly competing with those in the U.S. for workers.

For decades, the U.S. has relied on Mexican immigration to keep the agricultural sector afloat, Taylor says, but that won't continue forever. "Our era of farm labor abundance is coming to an end," he said.

Taylor thinks growers will eventually need to embrace a new approach: either grow less labor-intensive crops, seek out a new source of immigrant workers or improve technology. But for now, the most realistic option is a guest-worker program.

"If I were them, I would probably be pushing for it as a short-term stop-gap solution," he said, "but I would sure want to be aware of this longer-term trend in supply of labor from Mexico."

5. Agriculture Gets Special Treatment From the Federal Government

Unlike other sectors, like manufacturing or technology, farming has its own official seat in Washington: the U.S. Department of Agriculture (USDA). Founded 150 years ago when most Americans were farmers, the USDA now manages everything from school lunches to maintaining national forests.

Because of our agrarian roots -- and the importance of the food supply -- agriculture gets special consideration from the U.S. government, according to Philip Martin.

"We have always made exceptions for agriculture," he said. "We have unique agricultural policies, there's water that's different for agriculture, there are tax laws that are different for agriculture....It's history."